Question

Briefly describe the Escherichia coli (E. coli) lac operon and its regulation.

Answer 1

Lac Operon

The lac operon is an operon or group of genes with a single promoter (transcribed as a single mRNA). The genes in the operon encode proteins.

The lac operon of E. coli contains genes that are involved in lactose metabolism and transport of lactose. It is expressed only when lactose is present and glucose is absent.

Structure of lac operon

The lac operon contains three genes: lacZ, lacY, and lacA. These genes are transcribed as a single mRNA, under control of one promoter

• The lacZ gene encodes - an enzyme called ?-galactosidase, which is responsible for splitting lactose (a disaccharide) into glucose and galactose (monosaccharides).
• The lacY gene encodes - a membrane protein called lactose permease, which is a transmembrane "pump" that allows the cell to import lactose.
• The lacA gene encodes - an enzyme known as a transacetylase that attaches a particular chemical group to target molecules.

Regulators for operon-

• Two regulators turn the operon "on" and "off" in response to lactose and glucose levels:

1. the lac repressor

2. catabolite activator protein (CAP).

• The lac repressor acts as a lactose sensor. It normally blocks transcription of the operon, but stops acting as a repressor when lactose is present.

• Catabolite activator protein (CAP) acts as a glucose sensor. It activates transcription of the operon, but only when glucose levels are low.

Regulation of E. coli (lac operon)-

The type of regulation that the lac operon undergoes is referred to as negative inducible, meaning that the gene is turned off by the regulatory factor (lac repressor) unless some molecule (lactose) is added.

components required-

The gene for the Lac repressor. A fourth gene, the I gene, encodes the Lac repressor protein, it can block the expression of the Z, Y, and A genes.

The lac promoter site. The promoter is the site on the DNA to which RNA polymerase binds to initiate transcription.

The lac operator site. The operator is the site on the DNA to which the Lac repressor binds.

The I gene continually makes repressor. The repressor binds to the operator region, blocking the RNA polymerase bound to the promoter region from transcribing the adjacent structural genes.

When lactose is present, it binds to the repressor and changes its shape so that the repressor no longer binds to Operator. The RNA polymerase is then able to transcribe the Z, Y, and A structural genes, so the three enzymes are produced.